Application of the cascaded multilevel inverter as a shunt active power filter

Abstract unavailable please refer to PD

Direct control of D-STATCOM based on 23-level cascaded multilevel inverter using harmonics elimination pulse width modulation

The distribution static synchronous compensator (D-STATCOM) is primarily used for solving power quality problems. Normally, the phase-shifted pulse width modulation (PS-PWM) switching is employed in conjunction with the direct control of the D-STATCOM. However, the PS-PWM exhibits high switching losses. To alleviate this problem, a direct control scheme for D-STATCOM based on the harmonic elimination PWM (HEPWM) switching is developed. Due to the difficulty in solving the equations for the HEPWM angles, no work is reported on the direct control for a multilevel voltage source inverter (MVSI) D-STATCOM with more than 15-levels. Thus, the main contribution of the work is the application of HEPWM for 23-level cascaded MVSI using a wide modulation index (MI) range (i.e. 5.40 – 8.15 p.u). The main motivation to utilize the high number of level is to allow for the output voltage of the D-STATCOM to be sufficiently high, thus avoiding the use of step-up transformer. Furthermore, the achieved MI keeps the total harmonic distortion of the MVSI output voltage below the IEEE 519 Standard (5%) over the entire operating range. The eleven HEPWM switching angles were computed using an optimization technique, known as the differential evolution. Since the angles were computed offline, they were retrieved from a look-up table whenever the output voltage of the MVSI was to be constructed. The HEPWM-based direct control was benchmarked against the popular PS-PWM using ± 6.5MVAr/11kV D-STATCOM modelled in MATLAB-Simulink and PLECS software. For the same switching frequency, the proposed HEPWM switching exhibited superior harmonic spectra, hence had lower losses. Furthermore, the size of the series coupling inductor can be reduced to at least half. Dynamically, the steady state value of the reactive current was reached in less than one mains cycle when a transition from the full inductive to full capacitive modes was imposed. In addition, the proposed D-STATCOM controller mitigated the swell and sag problems in less than one cycle

Intelligent STATCOM Voltage Regulation using Fuzzy Logic Control

Reactive power compensation is a very important and challenging task in electrical power systems today. Future trends foreseen in power systems such as high interconnectivity and the integration of renewable energy resources produce even more issues related to power system control and stability. Flexible AC transmission systems are vastly used in power systems in order to mitigate several performance aspects found in typical power systems. One shunt connected device in particular, STATCOM, is very powerful and commonly used in voltage regulation at the power transmission level. STATCOM uses voltage sourced converters to inject or absorb reactive power from the power grid as commanded to stabilize the transmission line voltage at the point of connection. The control of STATCOM has relied historically on using traditional PI controllers, however, since the dynamic response of STATCOM highly affects its ability to perform its task, improving the capabilities of STATCOM using more advanced control approaches has become vital for both manufacturers and power systems operators. Fuzzy logic control, as one area of artificial intelligence techniques, has been emerging in recent years as a complement to the conventional methods in various areas of power systems control. The most significant advantage of fuzzy controller as an intelligent controller is that it doesn’t require mathematical modelling. It is robust and nonlinear in its nature, and expert’s knowledge can be utilized in generating control rules. The main contribution is to use fuzzy logic control theory to design a pure fuzzy logic control and another fuzzy adaptive PI control strategies for STATCOM that are superior in performance to traditional PI control approach. This will increase STATCOM’s ability to seamlessly perform their task in voltage regulation. This work investigates the performance of classical PI controlled STATCOM then compares it with fuzzy logic based STATCOM and fuzzy adaptive PI controlled STATCOM. Simulations done using MATLAB on a three generator test system show that adaptive fuzzy PI control technique is faster in responding to voltage variations and better in tracking the reactive current reference. Results also show that a direct control using fuzzy logic provides even faster voltage regulation and acts almost as a perfect tracker for reference reactive current

Control of Static Converters for Grid-Side and Machine-Side Applications

The research activities summarized in this Ph.D. thesis are mainly referred to the power electronics field, with some extensions related to electric machines and electrical drives. The first chapter focuses on the analysis and control of an unconventional static converter able to extend a common 1-phase mains feeder into a standard 3-phase power supply featuring either a 3-wires or a 4-wires output, the latter including neutral. Such converter exhibits a complete power reversibility and permits to achieve a good power quality level both at the input and the output side. It is proposed as an attractive alternative to conventional solutions possibly available in the market, such as converters for drives supplied by 1-phase mains yet using 3-phase motors, thanks to the following benefits: greater simplicity, lower cost, inherent active-filter-like operation at supply side, low harmonic distortion at load side. Such converter might be then successfully applied in any application requiring a 3-phase standard supply when a 1-phase mains feeder is available. A theoretical analysis of the converter is presented as well as a semi-ideal simulation model implemented referring to different control strategies. Several simulation results are finally reported and commented, confirming the effectiveness of the proposed solution. The second chapter focuses on the real-time control of 3-phase single-dc-bus shunt active filters employed for the parallel compensation of harmonics, reactive components and unbalancing in the currents drawn by a power supply when generic 3-phase non-linear, non-resistive and unbalanced loads are connected. In particular, the specific issues related to applications featuring a high fundamental frequency, such as in aerospace ambit, were addressed, investigating an innovative improved dead beat digital control algorithm. Such solution was proposed and get ready mainly aiming to achieve a rather high bandwidth of the current control loop and a good reference tracking even when the number of commutations per fundamental period that can be used is rather low. In order to probe the performances of the proposed control strategy, a simulation model was first developed and a prototype system was finally get ready. The results obtained from several virtual and experimental tests are reported and commented referring both to standard industrial and much more demanding aerospace operative conditions, thus proving the validity of the proposed solution. The third chapter focuses on the real-time control of 3-phase multilevel shunt active filters employing a multilevel cascaded H-bridges structure, again mainly referring to applications featuring a high fundamental frequency such as in aerospace ambit. In fact, such power structures may permit to improve the equivalent converter performances while keeping at relatively low values the actual switching frequency of the power devices. In particular, the combined application of an innovative modulation technique and of a dead-beat strategy analogous to the one described in the previous chapter was investigated. A theoretical analysis of the proposed control strategy is reported, as well as several experimental results obtained from a prototype system purposely get ready and tested at both industrial and aerospace frequency, highlighting the potential of the proposed solution especially for the latter applications. The experimental activities related to chapters 2 and 3 were developed during a study period spent at the University of Nottingham, UK. The fourth chapter deals with the modeling and control of an innovative rotary-linear brushless machine. In particular, after its ideal analytical modeling and operation principle, its basic control strategy inspired to sinusoidal brushless machines is presented, reporting some simulation results. A more detailed simulation model based on the equivalent magnetic circuit approach is then presented, permitting to approximately take into account several secondary aspects neglected by the simpler basic sinusoidal model while remaining not much computationally intensive as a finite element model would be. Simulation results obtained by such model are reported and commented, highlighting its potential usefulness for both preliminary machine design purposes and for analyzing the operation of a complete drive. Finally, the fifth chapter presents the application of the same intermediate-level modeling approach described in chapter 4 to a consequent-pole brushless machine featuring an unconventional magnet-pole angular width ratio. After some considerations on the specific arrangement examined, which was conceived to achieve a better exploitation of the active materials, a simulation model of the machine is presented and numerical results are reported and commented, highlighting the usefulness of the proposed intermediate-level modeling approach

Cost-Effective Model Predictive Control Techniques for Modular Multilevel Converters

In this thesis, model predictive control (MPC) techniques are investigated with their applications to modular multilevel converters (MMCs). Since normally a large number of submodule (SM) capacitor voltages and gate signals need to be handled in an MMC, the MPC schemes studied in this thesis are employed for determining only the voltage levels of converter arms, while gate signals are subsequently generated by the conventional sorting method. Emphasis is given to inner-loop current control in terms of phase current and circulating current, aiming at performance enhancement and computation reduction. A variable rounding level control (VRLC) approach is developed in this thesis, which is based on a modification of the conventional nearest level control (NLC) scheme: instead of the conventional nearest integer function, a proper rounding function is selected for each arm of the MMC employing the MPC method. As a result, the simplicity of the NLC is maintained while the current regulating ability is improved. The VRLC technique can also be generalized from an MPC perspective. Different current controllers can be considered to generate the arm voltage references as input of the VRLC block, thus refining the control sets of the MPC. Based on the decoupled current models, the accumulated effect of SM capacitor voltage ripples is investigated, revealing that the VRLC strategy may not achieve a proper performance if the accumulated ripple is nontrivial compared to the voltage per level. Two indexes are also proposed for quantifying the current controllability of the VRLC. Benefiting from this analysis, A SM-grouping solution is put forward to apply such MPC techniques to an MMC with a large number of SMs, leading to an equivalent operation of an MMC with much reduced number of SMs, which significantly increases the current regulating capability with reduced complexity. As an example, the SM-grouping VRLC proposal is analyzed and its system design principles are described. This thesis also develops another MPC technique which directly optimizes the cost function using quadratic programming technique. Both a rigorous and a simplified procedure are provided to solve the optimization problem. Compared with the conventional finite control set (FCS)-MPC method which evaluates all voltage level combinations, the proposed scheme presents apparent advantage in terms of calculation cost while achieving similar performance

Modular platform for research in microgrids

The present Ph.D. thesis has been developed following an Industrial Ph.D. program and verses on developing a commercial piece of equipment for teknoCEA, a spin-off company from CITCEA-UPC. The thesis is centered on developing power electronics-based emulation systems for research in microgrids. Lately, the use of power electronics-based emulation systems is drawing substantial attention in the field of microgrids because their characteristics substantially facilitate research in laboratory facilities. First, the suitability of different topologies for implementing an emulation platform is analyzed. The focus is set on the topologies adjustability to implement various types of emulation systems. The analysis determines the most appropriate number of legs for the platform. A comparative analysis is done between two-level and multi-level topologies to determine their suitability based on different aspects. Moreover, the analysis confirms the usefulness of wide-bandgap semiconductors for this type of application. Next, a control structure is proposed together with its implementation in a low-cost microcontroller based on a modular software architecture. The control strategy based on fractional proportional resonant controllers for AC emulation systems provides a control system with high control bandwidth while keeping a low computational cost. The control strategy for DC emulation systems is provided to reach a fast transient response and immunity to external disturbances, which is key for good emulation of electric systems. The modular software architecture provides a software framework easily adjustable to the needs of multiple emulation systems. That allows the implementation of the multiple control strategies with minimum changes. Additionally provides a graphical representation of the software architecture from a static and dynamic point of view. Last, the reliability of the proposed platform is assessed based on the reliability curves provided in the literature. The reliability analysis is centered on the semiconductors and capacitors. It provides evidence that emulation systems typical currents and voltages clearly affect their reliability. For the capacitors reliability assessment, a thermal modeling methodology is proposed to overcome the limitations of standard approximations. The methodology is based on anisotropic modeling of the capacitor winding. Finally, the reliability analysis establishes the guidelines to assess the platform reliability if a given mission profile is provided.La present tesi doctoral s'ha dut a terme seguint un programa de doctorat industrial. La tesi exposa el desenvolupament d'un equip comercial per a teknoCEA, una spin-off del CITCEA-UPC. La tesi es centra en el desenvolupament d'emuladors basats en electrònica de potència per recerca en el camp de les microxarxes. Darrerament, l'ús d'emuladors s'ha estès ja que les seves característiques faciliten molt la recerca en laboratoris. En primer lloc, s'analitza la idoneïtat de diferents topologies per implementar una plataforma d'emulació. El focus recau en la capacitat de diferents topologies per ajustar-se a la implementació de múltiples sistemes d'emulació. L'anàlisi determina el número òptim de branques. Un anàlisi comparatiu entre topologies dos nivells i multinivell permet determinar-ne la idoneïtat en funció de diferents aspectes. A continuació, es proposa una estructura de control juntament amb la seva implementació en un microcontrolador de baix cost a partir d'una arquitectura de programari modular. L'estratègia de control basada en controladors FPR (fractional proportional resonant) per a emuladors de corrent altern, proporciona un sistema de control amb un gran ample de banda amb un baix cost computacional. L'estratègia de control proposada per emuladors de corrent continu proporciona una resposta transitòria ràpida i elevada immunitat a pertorbacions, aspecte clau per a una bona emulació de sistemes elèctrics. L'arquitectura de programari modular proporciona un marc de programari fàcilment ajustable a les necessitats de múltiples emuladors. Això permet la implementació de les múltiples estratègies de control amb canvis mínims. A més, ofereix una representació gràfica de l'arquitectura del programari tant des d'un punt de vista estàtic com dinàmic. Finalment, s'avalua la fiabilitat de la plataforma a partir de les corbes de fiabilitat disponibles a la bibliografia científica. L'anàlisi es centra en els semiconductors i condensadors i proporciona evidència que els corrents i les tensions típics en emuladors afecten la seva fiabilitat. Per a l'avaluació de la fiabilitat dels condensadors, es proposa una metodologia de modelització tèrmica que permet superar les limitacions de les metodologies emprades típicament en la bibliografia científica. La metodologia es basa en el modelatge del bobinat del condensador com un element anisòtrop. Per últim, l'anàlisi de fiabilitat estableix les pautes per avaluar la fiabilitat de la plataforma en el cas que es proporcioni un perfil d'operació determinat.Postprint (published version

OPTIMAL PULSE WIDTH MODULATION OF MULTILEVEL INVERTERS FOR MEDIUM VOLTAGE DRIVES

Ph.DDOCTOR OF PHILOSOPH